Fuel assembly for nuclear reactors

ABSTRACT

A fuel assembly for nuclear reactors is formed of a plurality of substantially parallel fuel rods arranged between a top plate and a bottom member inside a sheathing tube. Spacer elements for these tubes are provided with leaf springs secured to the spacer frames, the free ends of the springs having buttons thereon engaging in holes in the sheathing tube to position the spacer elements in the tube.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority Aart Van Santen Hokasen;

Lars-Erik Samskog, Vasteras; Lars Tornblom, Vasteras, all 01 Sweden744,127

July 1 l, 1968 Sept. 21, 1971 Allmanna Svenska Elektriska AktiebolagetVasteras, Sweden July 17, 1967 Sweden FUEL ASSEMBLY FOR NUCLEAR REACTORS7 Claims, 1 1 Drawing Figs.

[52] US. Cl 176/78, 176/76 [51] Int. Cl G2lc 3/32 [50] Field of Search176/76, 78, 74, 81

References Cited UNITED STATES PATENTS Pierce, Jr

Frisch Fortescue et al. Bekkering et a1. ,1 Sankovich Pon Schmidt Hastamet al. Glandin Primary Examiner-Carl D. Quarforth AssistantExaminer-Gary Solyst Anorney.lennings Bailey, .lr.

220/46 176/78 176/78 X 176/78 176/78 X 176/76 X 176/76 X 176/76 X 176/78ABSTRACT: A fuel assembly for nuclear reactors is formed of a pluralityof substantially parallel fuel rods arranged between a top plate and abottom member inside a sheathing tube. Spacer elements for these tubesare provided with leaf springs secured to the spacer frames, the freeends of the springs having buttons thereon engaging in holes in thesheathing tube to position the spacer elements in the tube.

PATENTED SEPZI I971 SHEET 2 BF 3 INVENIUR N SA 'FE A/ AA RT VA LA RS'gR/K SAN\S moc- W MY PATENIED SEP21 l97| SHEET 3 OF 3 w RFMK. m s MV mmmw vFUEL ASSEMBLY FOR NUCLEAR REACTORS The present invention relates to afuel assembly for nuclear reactors in which a plurality of substantiallyparallel fuel rods are arranged inside a sheathing tube and between atop plate and a bottom member. The fuel rods are held at predetermineddistances from each other by means of spacer elements arranged in atleast one group surrounded by a spacer frame.

THE PRIOR ART In such fuel assemblies it is previously known to positionthe spacer group axially by attaching a few of the spacer elements inthe group between divided fuel rods. At these dividing points the fuelis replaced by threaded plugs, for instance of a zirconium alloy. Thismethod of positioning the spacers is a considerable economicdisadvantage because of the wasted" fuel length and due to the fact thatit involves a number of extremely expensive welding seams between thecladding of the fuel rods and the threaded plugs. Furthermore, eachwelding seam must be considered as a potential safety risk. It has alsobeen proposed to position the spacer groups axially by arranging longlongitudinal support members between them and between them and the endplates of the fuel assembly. This does not offer an economicallyacceptable solution either, since the material in these support membersis either expensive but does not absorb neutrons or is not so expensivebut instead absorbs neutrons to a considerably greater extent.

The method of axial positioning of the spacer groups is extremelyimportant, particularly for those boiling reactor fuel assemblies havingreusable sheathing tubes. To remove the sheathing tube from a usedassembly after some screws or the like have been loosened there must beno parts of the sheathing tube or fuel rod bundle which might engagewith each other and prevent separation or at least make it considerablymore difficult.

SUMMARY OF THE INVENTION One object of the present invention is toeffect a fuel assembly having a reliable but at the same time easilydetached attachment of a spacer element group surrounded by a spacerframe in the sheathing tube of the fuel assembly for axial positioningof the spacer group. This is made possible by having the spacer framecarry at least one spring means, preferably a resilient tongue, thespring action of which is directed towards the inside of the sheathingtube at at least one contact point, said spring means being providedwith a male or female member, preferably a male member in the form of ashort peg, to engage in a female or male member, respectively, arrangedin or on the wall of the sheathing tube, preferably a female member inthe form of a through-running hole somewhat larger than the peg. It issuitable to allow the resilient tongue directly or indirectly (forexample by means of a collar arranged on the short peg) to cover andsubstantially seal the hole in the sheathing tube.

A further object is to construct a fuel assembly having a reusablesheathing tube as well. This is made possible by the additionalconstructional steps that the sheathing tube is removably attached tothe top plate, preferably by means of locked screws, and axially movablewith respect to the bottom member. Therefore, if special support membersare not arranged in the fuel assembly, it is necessary to make some ofthe rods in the bundle capable of carrying the entire weight of theassembly during manipulation. In an alternative and more advantageousembodiment the bottom member consists of a separate bottom plate and aseparate transition part fitting into the fuel assembly plate (corebottom), the sheathing tube is detachably attached to the top plate,preferably by means of locked screws, and rigidly attached to thetransition part, preferably by means of rivets or locked screws, and thebottom plate rests on the transition part. In this case the sheathingtube carries the load. Even if during manipulation of the fuel rodbundle some individual fuel rods are permitted to carry the weight ofthis bundle, during manipulation of the fuel assembly the sheathing tubewill carry the entire weight of the assembly and take up all stresses.Under the assumption that the sheathing tube is not much deformed, theprobability of destroying fragile rods, which have been exposed toradiation, is zero in this embodiment during replacement of the fuel assembly.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be describedfurther with reference to the accompanying drawings.

FIG. 1 shows a fuel assembly with a weight carrying and reusablesheathing tube, partly in longitudinal section along the line I-I inFIG. 2.

FIG. 2 shows an upper end view of the fuel assembly in FIG. 1.

FIG. 3 shows a cross section of the fuel assembly according to the lineIll-III in FIG. 1.

FIG. 4 shows a lower end view of the fuel assembly in FIG. 1.

FIG. 5 shows an enlargement of part of FIG. 3.

FIG. 6 shows a spring arranged along the spacer frame.

FIG. 7 shows a spring arranged across the spacer frame.

FIG. 8 shows a fuel assembly having a reusable sheathing tube, partly inlongitudinal section along the line VlIl-VIII in FIG. 10.

FIG. 9 shows the central part of the fuel assembly according to FIG. 8,partly in longitudinal section along the line IXIX in FIG. 10.

FIG. 10 shows an upper end view of the fuel assembly in FIG. 8.

FIG. 11 shows a lower end view of the fuel assembly in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings I designates anumber of fuel rods contain ing a ceramic nuclear fuel such as U0 in theform of sintered pellets 2. These are enclosed in a cladding tube 3sealed at the ends by end plugs 4 and 5 or 6 and 7. The material in thecladding tube and end plugs may consist of a zirconium alloy such asZircaloy. In the upper part of each rod a fission gas space 8 isarranged containing a helical spring 9. The fuel rods are arrangedparallel to each other between a top plate 10 and a bottom member 11 andare surrounded by a sheathing tube 12 having substantially square crosssection. FIG. 1 shows an embodiment where the bottom member 11 consistsof a separate bottom plate 13 provided with legs 14 and a separatetransition part 15 fitting the assembly plate (core bottom). The legs 14rest on the transition part 15. The top plate 10 and the bottom plate 13are gridlike and provided with recesses to receive projections arrangedon the end plugs 4, 5, 6 and 7 to position the fuel rods atpredetermined distances from each other.

So that the fuel rods, which may exceed 3.5 m. in length, for instance,and may be 12.5 mm. wide, for instance, are not destroyed by vibrationscaused by the coolant flowing through the fuel assembly, spacer elementgroups 16 are arranged at several levels (only one level is shown), tokeep the fuel rods in the position intended. Each of these spacerelement groups 16 comprises a number of spacer elements 17 arranged in aspacer frame 18. The spacer elements 17 may, for instance, be shaped asshown in the French Pat. No. 1,487,538 or, even more advantageously, asshown in our copending US. Pat. application Ser. No. 693,412. The spacerframe 18 is centered in the sheathing tube I2 by means of spring meanscarried by the frame 18, for example leaf springs in the form of springtongues 19 to a number of eight, thus in the case shown two springs foreach side of the sheathing tube. In order to position the spacer elementgroup I6 axially in relation to the sheathing tube 12, the springtongues 19 which are arranged on two opposite sides of the spacer frame18 are provided with male members in the form of short pegs or pins 20intended to engage in female members arranged on the sheathing tube inthe form of holes 21, somewhat larger than the pins. These holes 21 aresuitably drilled with the help of a jig and it is also suitable for thespring tongue 19 to be shaped so that it covers the somewhat too largehole 21 and thus limits leakage. FIGS. 5, 6, 7 and 9 show suitableembodiments of the spring tongue 19 and its attachment to the spacerframe 18.

The combination of a plurality of fuel rods with spacers and top andbottom plates is usually quite simply called a bundle. In the fuelassembly according to FIGS. 1-4 the bundles comprises 64 fuel rods, onlytwo of which support the bundle. Of course a different number of rods,for example four, may be made supporting if desired.

The end plugs 6 and 7 of the supporting rods have thicker and longerprojections which are threaded at the outer ends and can be locked bymeans of nuts 22 on the other side of the top and bottom plates,respectively, it being ensured that there is sufficient clearance forthermal expansion between the nuts 22 and the plates 10 and 13,respectively, and between the top plate 10 and the upper shoulders 23 ofthe rods 1. The low number of supporting rods is possible since theirsupporting function is only made use of during manipulation of thebundle, for example when inserting a bundle in the sheathing tube. Whenthe assembly is being manipulated, the lifting force passes through ahandle 24 attached to the top plate 10, through the top plate and thesheathing tube 12 attached to the top plate by means of countersunkscrew 25, to the transition part attached to the sheathing tube byrivets 26 or screws 27, on which transition part, as mentioned before,the bottom plate 13 rests, and thus also the bundle. The screws 25 maybe locked by punch marks, HeliCoilQMidGrip, or the like. When the fuelassembly is being taken out of the core there is no risk that the rods1, which are fragile due to the radiation, will break, with theresultant serious consequences. A fuel assembly can be taken apart sothat the sheathing tube 12 and transition part 15 can be used again,after the screws 25 which connect the sheathing tube and top plate havebeen loosened and the pins of the springs 19 have been pressed in linewith the inside of the sheathing tube 12. This can be achieved with thehelp of plates of the like provided with pins and pressed against thesides of the sheathing tube. If it is feared that the few supportingrods holding the bundle together will not allow the bundle to be drawnout of the sheathing tube without breaking, it is possible simply toturn the fuel assembly and shake out the bundle. The elongatedlongitudinal springs 23 arranged at the upper end of the fuel assemblyserve to center the fuel assembly in the upper core grid and may beattached to the top plate 10 or to the sheathing tube 12, and in thelatter case by reused together with this. In neither case need they betaken away when the fuel assembly is taken apart.

The fuel assembly according to FIG. 8 also has a reusable sheathing tubebut differs from the fuel assembly according to FIG. 1 mainly in thatthe sheathing tube 12 does not have a long supporting function. In abundle containing 64 rods 1 in this case, for example eight are made togive support and these eight are attached at the lower end to the bottommember 11. The sheathing tube 12 is screwed to the top plate 10 butsurrounds only a part of the bottom member 11 without being attached toit. The lower edge of the sheathing tube 12 therefore has somewhatlimited axial movement in relation to the bottom member 11. Thenonsupporting rods 1 in the bundle are provided with prestressed helicalsprings 29 arranged between the upper stops 23 of the rods 1 and the topplate 10. These helical springs 29 carry the sheathing tube 12.Otherwise there are no essential differences between the fuel assembliesaccording to FIG. 1 and FIG. 8. For example in both cases the sheathingtube 12 can be detached from the fuel assembly when some screws (25)have been loosened and the pins 20 pressed in.

The above description, as well as the accompanying drawings, is intendedto illustrate the general principle of the invention and severalembodiments are feasible within the scope of the following claims,particularly with respect to the detailed design and l putting togetherof the separate arts of the fuel assembly. or examp e the above methodor axial positioning of the spacer groups can also by used for fuelassemblies having sheathing tubes which are not reusable, male andfemale members may be reversed and a number of other designs may be usedfor these members apart from holes and pins.

It is claimed:

1. A fuel assembly for nuclear reactors comprising a plurality ofsubstantially parallel fuel rods arranged between a top plate and abottom member inside a sheathing tube, separated at predetermineddistances from each other by spacer elements arranged within thesheathing tube in at least one group surrounded and supported by aspacer frame, wherein the improvement comprises that the spacer framecarries at least one spring means, the resilient spring action of whichis directed towards the inside wall of the sheathing tube at least onecontact point, said spring means and said wall being provided withinterengaging male and female members, wherein the male member consistof a short peg and the female member of a hole somewhat larger than thepeg, to position the spacers axially in the sheathing tube.

2. A fuel assembly according to claim 1, wherein the hole is arranged inthe wall of the sheathing tube and the peg on the spring means.

3. A fuel assembly according to claim 1, wherein the spring meanscomprises a resilient tongue.

4. A fuel assembly according to claim 1, wherein the sheathing tube isdetachably attached to the top plate and axially movable with respect tothe bottom member.

5. A fuel assembly according to claim 1, wherein the bottom membercomprises a separate bottom plate and a separate transition part whereinthe sheathing tube is detachably attached to the top plate and rigidlyattached to the transition part on which the bottom plate rests.

6. A fuel assembly according to claim 5, wherein the sheathing tube isrigidly connected to the transition part.

7. A fuel assembly according to claim 4, wherein the sheathing tube isscrewed to the top plate.

2. A fuel assembly according to claim 1, wherein the hole is arranged inthe wall of the sheathing tube and the peg on the spring means.
 3. Afuel assembly according to claim 1, wherein the spring means comprises aresilient tongue.
 4. A fuel assembly according to claim 1, wherein thesheathing tube is detachably attached to the top plate and axiallymovable with respect to the bottom member.
 5. A fuel assembly accordingto claim 1, wherein the bottom member comprises a separate bottom plateand a separate transition part wherein the sheathing tube is detachablyattached to the top plate and rigidly attached to the transition part onwhich the bottom plate rests.
 6. A fuel assembly according to claim 5,wherein the sheathing tube is rigidly connected to the transition part.7. A fuel assembly according to claim 4, wherein the sheathing tube isscrewed to the top plate.